The present invention relates to gain control and, in particular, to a digital gain control.
For over half a century, television signals have been broadcast in accordance with standardized analog signal formats, such as the NTSC format in the United States, the PAL format in Europe, and the SECAM format in France. Analog television signal formats have inherent limitations that preclude further improvement of picture quality, however, those inherent limitations can be overcome by broadcasting television signals in a digital format. Happily, advances in the design and processing of digital integrated circuits of ever increasing complexity and capability has resulted in sophisticated digital signal processing becoming available in a practical form and at reasonable cost.
Present digital television signal broadcast formats include the Digital Video Broadcast (DVB) format in use in Europe and the Advanced Television Standards Committee(ATSC) formats, such as ATSC format A/53 in use in the United States. Unfortunately, the various systems proposed and/or adopted for transmission of television signals in digital format are not compatible with the existing processing of television signals in analog format. This incompatibility arises not only because one format is analog and the other digital, but also because the signal processing at radio frequencies (RF) and/or at intermediate frequencies (IF) required to properly receive the formats are also different, such as in required filtering. Filtering at IF is typically implemented in surface acoustic wave (SAW) filters and SAW filters for NTSC signals should have a vestigial-sideband filter characteristic and those for ATSC signals should have a root-cosine filter or a xe2x80x9croofingxe2x80x9d (flat) filter characteristic.
The conventional approach to a receiver for receiving television signals in both analog and digital formats have required dual SAW filters and typically dual IF amplifiers, one set for each format, thereby undesirably increasing the complexity and cost of the television receiver and possibly introducing electrical interference. Such prior art arrangements typically employ the same conventional analog automatic gain control (AGC) as is employed in conventional analog format receivers. Such receivers often include two separate analog-to-digital (A/D) converters, one for the analog format signals and one for the digital format signals. Unfortunately, such analog AGC and duplicative circuitry also adds undesirable cost and complexity to the receiver which is coming to include increasing digital processing and control functions.
In the United States, for example, the change over from all-NTSC analog format broadcasting to all-ATSC digital format broadcasting will be phased in gradually over many years so as not to instantly obsolete the installed base of existing NTSC television transmitters and receivers. Initially, only a few of the available programs may be in digital format and only a few of the broadcast channels will be changed from NTSC analog format to ATSC digital format, and later additional programs and channels will be converted. This suggests a further problem in that the conventional fixed analog AGC does not respond to the particular attributes of ATSC and NTSC formatted signals in view of the unknown and unpredictable program and channel broadcast patterns in any particular geographical area.
Accordingly, there is a need for a receiver having a digital gain control. In addition, it is desirable that such receiver receive both analog format and digital format signals and, if necessary, adapt the gain control characteristics responsive to the received signal, for example, whether the received signal is in analog format or in digital format.
To this end, a gain control according to the present invention comprises means for converting an output signal from a gain-controllable circuit into digital data and a first digital comparator comparing a representation of the magnitude value of the digital data to a digital reference value to produce a digital error value. A generator generates an output signal responsive to the digital error value, and a low-pass filter responsive to the output signal applies a gain control signal to the gain-controllable circuit.
According to another aspect of the present invention, a method of controlling the magnitude of a signal provided by a controllable circuit comprises:
converting the signal magnitude to a sequence of digital words;
comparing a representation of the magnitude value of the digital words to a digital reference value to provide a digital error value;
generating an output signal responsive to the digital error value;
filtering the output signal to apply a control signal to the controllable circuit to control the magnitude of the signal provided thereby.